Sentinel 3: Homeworld Gets A Massive Update

A recent update to Sentinel 3: Homeworld has added a whole new campaign to the sci-fi tower defense game. Along with this 1.2.5 update the game has been slashed in price to $.99. Sentinel 3 is also a universal app that offers some amazing visuals with the setting being in deep space.
Even before this update, Sentinel 3 really set the bar for App Store tower defense games, and not just in the visual department. The game builds upon a successful formula of previous titles in the series and adds a commander unit that can be upgraded and moved around the battlefield.
The 1.2.5 update's new campaign (purchased through an IAP) adds more maps for players to defend, new weapons to use and new enemies to face. In addition to these new features, a new commander unit has also been included in the update. Here is a list of the various new features added (from the app description):

A whole new 15 map campaign available for purchase: Features a new commander unit, new enemies, new weapons & turrets and new map features and graphics! This also adds 15 endless maps and 6 new classic maps.

Use any already unlocked weaponry from the first campaign to help with the second.

The first two maps and one classic map from the new campaign are available for FREE to whet your appetites!

Shortcut purchases have been added for players who want to unlock everything instantly or attain a maximum level commander.

iPad level backgrounds are now higher resolution

Endless games now give you far more credits/XP

Many visual effects have been improved

Various bugfixes and optimizations

Tower defense games revolve around the concept of defensive gameplay where the player builds towers and other structures to stop incoming enemy hordes from overrunning the player's base.
For games in the genre to stand out, they have to be balanced. They should offer gameplay based on strategic planning rather than just exploitation of a single structure or attack against the enemy. Sentinel 3 has a nice balance of strategic gameplay and fast-paced action.
Sentinel 3: Homeworld (universal) can be purchased on the App Store for $.99.